The greenhouse effect from CO2 has recently been pointed out as one of the causes of the global warming phenomenon. Immediate actions are needed to protect the earth's environment. CO2 sources include human activities of burning fossil fuels, and there is an increasing demand for emission control. As a result, new construction of thermal power plants and the like using large amounts of fossil fuels has been stagnating because of high CO2 emissions.
Demand is increasing for power generation methods using renewable energies that produce no CO2, such as solar light, solar heat, wind power, geothermal heat, and tidal power. Of these, power generation systems using geothermal steam and geothermal water have been commercialized since the 1950s. With high construction costs, geothermal power plants used to decline in the age of decreasing fossil fuel costs, whereas the demand has been increasing again in recent years. Some of existing geothermal power plants are shifting from a flash geothermal power generation system in which a steam turbine is driven by geothermal steam to a binary geothermal power generation system in which hot water is used as a heat source to evaporate an organic working medium for generation because the thermal energy of the geothermal steam decreases gradually.
Such a binary geothermal power generation system uses a medium having a boiling point lower than that of water as the working medium. Examples of the low-boiling medium include chlorofluorocarbons which were used as the working medium of refrigerators until 1990. Since existing chlorofluorocarbons harm the ozone layer and there has been found no low-boiling medium to be a workable alternative, the binary geothermal power generation system has not been actively put to practical use in Japan.
Under the circumstances, binary power generation systems using flammable but produced-in- volume butane (C4H10) or pentane (C5H12) as the working medium have been commercialized.
In a technique disclosed in Patent Document 1, a pressure reducing steam-liquid separator flashes and separates geothermal water into steam and hot liquid water. The hot water having lower enthalpy preheats the working medium, and the flashed steam evaporates the working medium. Such a system is effective if the proportion of the flashed steam is small.
When organic working vapor medium is expanded in a turbine, the degree of superheat increases and gas (vapor) having a temperature higher than a condensation temperature in a condenser is condensed. In a technique disclosed in Patent Document 2, preheater outlet liquid medium is injected into an intermediate stage of a working medium turbine so that the gas having a high degree of superheat is mixed with the saturated liquid. As a result, the energy of the degree of superheat can be used to increase the driving flow rate of the turbine and improve the cycle efficiency.
In a technique disclosed in Patent Document 3, a steam turbine is driven by flashed steam from a geothermal water pressure reducing steam-liquid separator. The exhausted steam evaporates a medium, and hot water separated from the pressure reducing steam-liquid separator superheats the medium. Proposed modifications include the following:
(1) Install a regenerator at the outlet of the medium turbine.
(2) Provide a two-stage medium turbine, and reheat the vapor medium by the hot water from the outlet of a superheater.
In a technique disclosed in Patent Document 4 and a technique disclosed in Patent Document 5, a steam turbine is driven by part of flashed steam from a geothermal water pressure reducing steam-liquid separator. The rest of the flashed steam evaporates a medium. The exhausted steam from the steam turbine and hot water from the pressure reducing steam-liquid separator preheat the medium. A regenerator is arranged at the outlet of a medium turbine. A modified embodiment is disclosed in which a two-stage medium turbine is provided and the vapor medium is reheated by hot water from the outlet of a superheater.
A technique disclosed in Patent Document 6 deals with a system that is not limited to geothermal power generation but also takes into account solar heat and the exhaust heat of thermal power generation etc. Two types of media, one for high temperature and the other for low temperature, are used to constitute a cascaded Rankine cycle, which is a basic form of cascade type.
Patent Document 7 discloses one including a plurality of evaporators with different pressures.